Contemporary interactive online map services use an optimization to package large map images so that web browsers can fetch just the parts of the image with which the human user is interacting. A single logical image is divided into millions of similarly-sized tiles that fit together seamlessly, and the tiles are small enough that each tile can be fetched rapidly. The browser covers a constant number of tiles so no matter how the view moves over the underlying image, the browser needs only to fetch a constant amount of data to draw the desired view. In other words, the browser software stitches together just enough tiles to show the user the part of the image visible on-screen. As the user scrolls or pans the view left, right, up, or down, the browser software computes exactly which portion of the image is required, and fetches additional tiles to fill in the newly-visible regions of the underlying image.
While this technique is applicable to any large image, it is especially clever for maps. Prior to its introduction, most online maps were drawn using a local projection. As the user scrolled the map, the browser requested that the mapping server draw a new map centered on the new location. By using a projection that was a function of the center of the map, such systems could present minimally-distorted maps, tailored to the specific view. On the other hand, due to the properties of projecting the ellipsoidal Earth into two dimensions, such views could not stitch together into a single continuous view. Consequently, every change in perspective by the user demanded a new rendering at the server. This imposes a high run-time cost on the server, limited by how elaborate server rendering can be, and the high round-trip latency provided a less-interactive experience to the user.